CHAPTER 16

Evolution: Speciation

16.1 Genetic Equilibrium•The Concept of Species•Variation of Traits in a Population•Allele Frequencies and Genetic Equilibrium•The Hardy – Weinberg Principle

16.2 Disruption of Genetic Equilibrium•Mutation•Migration•Genetic Drift•Natural Selection

16.3 Formation of Species •Isolated Populations•Rates of Speciation•Extinction

Evolution: Speciation

GUILLANO, JOSHUA P.

KADUSALE, CRISELDA

BABOR, AFRILOU

ARQUISAL, CHONA

CADAYONA, SHENELYN

REPORTERS

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16.1GENETIC

EQUILIBRIUM

The idea that the diversity of nature is divisible into a finite number of definable species. In general, species concepts grow out of attempts to understand the very nature of biological organization above the level of the individual organism.

The word species derived from the a Latin word “kind” or “appearance.” We use the concept of the species because we naturally tend to group together things that look alike.

THE CONCEPT OF SPECIES

The morphological species concept is the most intui t ive of the possible concepts and is also the oldest . I t stems basical ly from the idea that species are groups which are constant in appearance, which, when we f i rst look around in nature, seems qui te plausible. The morphological d i f ferences between each species al low us to dist inguish them from each other, a l ion from a t iger, an oak from a daisy etc. Carolus Linnaeus used this concept to catalogue the diversi ty of l i fe in his ‘Systema Naturae’ , and gave us the binomial name with which we st i l l at tempt to classi fy al l the organisms that l ive around us.

THE MORPHOLOGICAL CONCEPT OF SPECIES

The biological species concept is the most widely accepted species concept. It defines species in terms of interbreeding. For instance, Ernst Mayr defined a species as follows: "species are groups of interbreeding natural populations that are reproductively isolated from other such groups."The biological species concept explains why the members of a species resemble one another, i.e. form phenetic clusters, and differ from other species.

When two organisms breed within a species, their genes pass into their combined offspring. As this process is repeated, the genes of different organisms are constantly shuffled around the species gene pool. The shared gene pool gives the species its identity. By contrast, genes are not (by definition) transferred to other species, and different species therefore take on a different appearance.

This explanation should be compared with that given by the ecological species concept.Interbreeding between species is prevented by isolating mechanisms.Many biologists, including Richard Dawkins, define a species by the biological species concept.

VARIATION OF TRAITS IN A POPULATION

The niche variation hypothesis is an adaptive explanation for variation within populations and for, the differences in variation between populations in morphological, physiological or behavioral traits. It has received only partial support from empirical tests and has been criticized on theoretical grounds. Recent quantitative genetic models have made an advance by exploring the effects of mutation, migration, mating pattern and selection on phenotypic variance.

These models are reviewed and their most important features are integrated in a new model. In this model population variation is in a state of balance between the opposing forces of mutation and immigration, which tend to elevate variation, and selection and possibly genetic drift tending to decrease it.

Many traits are controlled by two or more genes and are called polygenic traits.

One polygenic trait can have many possible genotypes and phenotypes.

Height in humans is a polygenic trait.

Allele Frequencies

The number of times that the allele occurs in a gene pool, compared with the number of times

other alleles for the same gene occur

In genetic terms, evolution is any change in the relative frequency of alleles in a population

The allele for a widow’s peak is dominant over the allele for a hairline with no peak.

However, the presence of a widow’s peak may be less common in a population.

In real populations, phenotypic ratios are determined by the frequency of alleles as well as by whether the alleles are dominant or recessive.

When allele frequencies change

due to migration of a small subgroup of a

population it is known as th e founder effect.

Genetic equilibrium

The situation in which allele frequencies remain constant

Evolution as Genetic Change

Remember that evolution is any change over time in the relative frequency of alleles in a population.

This reminds us that it is populations, not individual organisms that can evolve overtime

5 conditions are required to maintain genetic equilibrium

1. There must be random mating

2. The population must be very large

3. There can be no movement into or out of the population

4. No mutations

5. No natural selections

Hardy – Weinberg principle

States that allele frequency in a

population will remain constant unless one or

more factors cause these frequencies to

change.

16.2DISRUTIVE GENETIC

EQUILIBRIUM

MutationsAny change in a sequence of DNA

Mutations can occur because of

Mistakes in DNA replication

Radiation or chemicals in the environment

Some mutations don’t affect the phenotype but some do

Migration is the relatively long-distance movement of individuals, usually on a seasonal basis. It is a ubiquitous phenomenon, found in all major animal groups, including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans.

The trigger for the migration may be local climate, local availability of food, the season of the year or for mating reasons.[2] To be counted as a true migration, and not just a local dispersal or irruption, the movement of the animals should be an annual or seasonal occurrence, such as birds migrating south for the winter, or a major habitat change as part of their life, such as young Atlantic salmon leaving the river of their birth when they have reached a few inches in size.

MIGRATION

MIGRATION

Migration is the relatively long-distance movement of

individuals

Genetic DriftThese individuals may carry

alleles in different relative frequencies than did the larger population from which they came

If so, the population that they found will be genetically different from the parent population

This cause is not natural selection, but chance

Genetic DriftNatural Selection is not the only source of evolutionary change

In small populations, an allele can become more or less common by chance

Genetic Drift

Descendants

Population A Population B

Natural SelectionDuring the…

Rainy season – enough food for everyone, no competition

Dry season – some foods become scarce

At that time, differences in beak sizes can mean the difference between life and death

Birds become feeding specialists

Natural selection and chance events can change the relative frequencies of alleles in a population and lead to

speciation.Speciation is the formation of new

species.A species is a group of organisms that breed with one another and

produce fertile offspring.

Natural SelectionThe Grants discovered that individual birds with

different size beaks had different chances of survival during a drought

Testing Natural Selection in Nature

Darwin hypothesized that finches had descended from a common ancestor and overtime, natural selection shaped the beaks of

different bird populations as they adapted to eat different foods

The Grants, realized that Darwin’s hypothesis relied on two testable

assumptions

Testing Natural Selection in NatureQ: Can evolution be observed in nature?

A: YES

1. Stabilizing Selection

When individuals near the center of

the curve have higher fitness than individuals at either

end of the curve

2. Directional Selection

When individuals at one end of the curve have higher fitness than individuals in the middle or

at the other end, directional selection takes place.

The range of phenotypes shifts as some individuals survive and reproduce while others do not.

3. Disruptive Selection

When individuals at the upper and lower ends of the curve have higher fitness than

individuals near the middle

Can create 2 distinct phenotypes

Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection.

The sexual struggle is of two kinds; in the one it is between individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive; whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners.

SEXUAL SELECTION

16.3 FORMATION Of SPECIES

Isolated Populations

A group of individuals of the same species that interbreed

Because members of a population interbreed, they share a common group of genes called a gene pool

Geographical Isolation

When two populations are separated by

geographic barriers such as rivers,

mountains, or bodies of water

Speciat ion in Darwin's Finches

Some birds from species A cross to a second island.

The two populations no longer share a gene pool.

GEOGRAPHIC ISOLATION

Isolating Mechanisms

Geographic isolation occurs when two populations are separated by

geographic barriers such as rivers or mountains.

Speciat ion in Darwin's Finches

Reproductive Isolation

If population B birds cross back to the first island, they will not mate with birds from population A.Populations A and B are

separate species.

Rates of origination have declined through the Phanerozoic. This appears to have been largely a function of sorting among higher taxa (especially classes), which exhibit characteristic rates of speciation (and extinction) that differ among them by nearly an order of magnitude. Secular decline of origination rates is hardly constant, however; many positive deviations reflect accelerated speciation during rebounds from mass extinctions.

There has also been general decline in rates of speciation within major taxa through their histories, although rates have tended to remain higher among members in tropical regions. Finally, pulses of speciation appear sometimes to be associated with climate change, although moderate oscillations of climate do not necessarily promote speciation despite forcing changes in species' geographical ranges.

RATES OF SPECIATION

Data from palaeontology and biodiversity suggest that the global biota should produce an average of three new species per year. However, the fossil record shows large variation around this mean.

Albert & Kaibab Squirrels

In biology and ecology, extinction is the end of an organism or of a group of organisms (taxon), normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point. Because a species' potential range may be very large, determining this moment is difficult, and is usually done retrospectively.

This difficulty leads to phenomena such as Lazarus taxa, where a species presumed extinct abruptly "re-appears" (typically in the fossil record) after a period of apparent absence.Through evolution, new species arise through the process of speciation—where new varieties of organisms arise and thrive when they are able to find and exploit an ecological niche—and species become extinct when they are no longer able to survive in changing conditions or against superior competition.

EXTINCTION

END OF CHAPTER :]

JOSHUA GUILLANO AND

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